Broadband arbitrary wavelength multichannel laser source

ABSTRACT

A multi-channel laser source, including: a bus waveguide coupled, at an output end of the bus waveguide, to an output of the multi-channel laser source; a first semiconductor optical amplifier; a first back mirror; a first wavelength-dependent coupler, having a first resonant wavelength, on the bus waveguide; a second semiconductor optical amplifier; a second back mirror; and a second wavelength-dependent coupler, on the bus waveguide, having a second resonant wavelength, different from the first resonant wavelength. In some embodiments the first semiconductor optical amplifier is coupled to the bus waveguide by the first wavelength-dependent coupler, which is nearer to the output end of the bus waveguide than the second wavelength-dependent coupler, the second semiconductor optical amplifier is coupled to the bus waveguide by the second wavelength-dependent coupler, and the first wavelength-dependent coupler is configured to transmit light, at the second resonant wavelength, along the bus waveguide.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. patent applicationSer. No. 17/327,508, filed May 21, 2021, entitled “BROADBAND ARBITRARYWAVELENGTH MULTICHANNEL LASER SOURCE”, which is a continuation of U.S.patent application Ser. No. 17/172,033, filed Feb. 9, 2021, entitled“BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASER SOURCE”, which is acontinuation of U.S. patent application Ser. No. 17/104,929, filed Nov.25, 2020, entitled “BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASERSOURCE”, which is a continuation of U.S. patent application Ser. No.17/022,901, filed Sep. 16, 2020, entitled “BROADBAND ARBITRARYWAVELENGTH MULTICHANNEL LASER SOURCE”, which is a continuation of U.S.patent application No. Ser. 16/007,896, filed Jun. 13, 2018, issued asU.S. Patent No. 10,811,848, issued on Oct. 20, 2020, entitled “BROADBANDARBITRARY WAVELENGTH MULTICHANNEL LASER SOURCE”, which claims thebenefit of U.S. Provisional Application No. 62/519,754, filed Jun. 14,2017, entitled “BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASERSOURCE”, and of U.S. Provisional Application No. 62/548,917, filed Aug.22, 2017, entitled “BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASERSOURCE”. The entire contents of all of the applications identified inthis paragraph are incorporated herein by reference.

FIELD

One or more aspects of embodiments according to the present disclosurerelate to light sources, and more particularly to a multichannel lightsource.

BACKGROUND

Some systems may use a silicon photonics integrated multichannel tunablelaser source which has arbitrary wavelength channels covering awavelength span of multiple hundreds to one or more thousands ofnanometers, a span which is much larger than the gain bandwidth of asingle reflective semiconductor optical amplifier (RSOA) die, or RSOA“chip”.

Such a laser source may be constructed using a single laser cavity foreach channel, with, e.g., each laser constructed according to U.S. Pat.No. 9,270,078 (the “'078 Patent”), which is incorporated herein byreference in its entirety. The channels may be combined externally tothe cavity using a channel combiner such as an optical multiplexer(MUX), or additional ring resonator tunable filters. The use of a MUXexternal to the laser cavities for combining may have the disadvantagesof (i) imposing a minimum channel spacing due to the periodicity of thepassband response, (ii) introducing stop-bands where channels cannotexist, and (iii) for small channel spacing (and a large number ofchannels), incurring relatively high MUX losses. In an embodiment withmultiple MUXs of different designs and channel spacings connected to oneor more additional MUX to combine the outputs of the multiple MUXs toone common output, the optical loss may also be relatively high.

Ring resonator tunable filters may be used to combine the light frommultiple lasers externally to the laser cavities, but such an embodimentincludes further ring resonator tunable filters in addition to the onesused inside each laser cavity. As a result, more tunable elements may beincluded, requiring more stabilization circuits. Moreover, locking theexternal filter wavelength to the internal laser filter wavelength mayincrease the complexity of the system.

Thus, there is a need for an improved multichannel laser source.

SUMMARY

According to an embodiment of the present disclosure there is provided amulti-channel laser source, including: a bus waveguide coupled, at anoutput end of the bus waveguide, to an output of the multi-channel lasersource; a first semiconductor optical amplifier; a first back mirror; afirst wavelength-dependent coupler having a first resonant wavelength; asecond semiconductor optical amplifier; a second back mirror; and asecond wavelength-dependent coupler having a second resonant wavelength,different from the first resonant wavelength; the first semiconductoroptical amplifier including: a first end coupled to the first backmirror, and a second end, the first wavelength-dependent couplerincluding: a channel port connected to the second end of the firstsemiconductor optical amplifier; a bus output connected to a firstportion of the bus waveguide; and a bus input, connected to a secondportion of the bus waveguide more distant from the output end of the buswaveguide than the first portion of the bus waveguide; the secondsemiconductor optical amplifier being coupled to the bus waveguidethrough the second wavelength-dependent coupler, the firstwavelength-dependent coupler being nearer to the output end of the buswaveguide than the second wavelength-dependent coupler, the firstwavelength-dependent coupler being configured to transmit light, at thesecond resonant wavelength, from the bus input of the firstwavelength-dependent coupler to the bus output of the firstwavelength-dependent coupler.

In one embodiment, the multi-channel laser source includes an outputcoupler at the output end of the bus waveguide, wherein the firstwavelength-dependent coupler is configured to transmit light at thefirst resonant wavelength from the channel port of the firstwavelength-dependent coupler to the bus output of the firstwavelength-dependent coupler.

In one embodiment, the first wavelength-dependent coupler is configuredto reflect a first portion of light received at the first resonantwavelength at the channel port of the first wavelength-dependentcoupler, and to transmit, to the bus output of the firstwavelength-dependent coupler, a second portion of light received at thefirst resonant wavelength at the channel port of the firstwavelength-dependent coupler.

In one embodiment, the first portion is at least 10% of the lightreceived, and the second portion is at least 40% of the light received.

In one embodiment, the first wavelength-dependent coupler is configuredto transmit, to a fourth port of the first wavelength-dependent coupler,light received at the channel port at the second resonant wavelength.

In one embodiment, the fourth port of the first wavelength-dependentcoupler is connected to an optical absorber.

In one embodiment, the first back mirror and the first semiconductoroptical amplifier are configured as a reflective semiconductor opticalamplifier.

In one embodiment, the first wavelength-dependent coupler includes afirst ring resonator.

In one embodiment, the first wavelength-dependent coupler furtherincludes a second ring resonator, the first ring resonator and thesecond ring resonator being configured to operate as a vernier ringresonator filter.

In one embodiment, the first wavelength-dependent coupler includes agrating assisted co-directional coupler.

In one embodiment, the first wavelength-dependent coupler furtherincludes a distributed Bragg reflector connected in cascade with thegrating assisted co-directional coupler.

In one embodiment, the first wavelength-dependent coupler includes awavelength actuator for adjusting the first resonant wavelength.

In one embodiment, the multi-channel laser source includes a phaseshifter between the first back mirror and the first wavelength-dependentcoupler.

In one embodiment, the multi-channel laser source includes an amplitudemodulator between the first back mirror and the firstwavelength-dependent coupler.

In one embodiment, the first semiconductor optical amplifier is the samesemiconductor optical amplifier as the second semiconductor opticalamplifier.

In one embodiment, the first semiconductor optical amplifier includes afirst waveguide in a first semiconductor chip and the secondsemiconductor optical amplifier includes a second waveguide in the firstsemiconductor chip.

In one embodiment, the first semiconductor optical amplifier includes awaveguide in a first semiconductor chip, and the second semiconductoroptical amplifier includes a waveguide in a second semiconductor chip,different from the first semiconductor chip.

In one embodiment, the multi-channel laser source includes: a wavelengthsensor configured to receive a portion of, and to sense a wavelength of,light emitted by the first semiconductor optical amplifier; and acontrol system configured: to receive a wavelength sensing signal fromthe wavelength sensor, to calculate a difference between the wavelengthsensing signal and a wavelength setpoint, and to apply a wavelengthcorrection signal to a wavelength actuator, to reduce the differencebetween the wavelength sensing signal and the wavelength setpoint.

In one embodiment, the multi-channel laser source includes a phaseshifter between the first back mirror and the first wavelength-dependentcoupler, wherein the wavelength actuator includes the phase shifter.

In one embodiment, the first wavelength-dependent coupler includes acoupler wavelength actuator for adjusting the first resonant wavelength,wherein the wavelength actuator includes the coupler wavelengthactuator.

In one embodiment, the wavelength sensor is configured to receive lightfrom a fourth port of the first wavelength-dependent coupler.

In one embodiment, the wavelength sensor includes a Mach-Zehnderinterferometer having a first arm and a second arm, longer than thefirst arm, and a temperature control system configured to control thetemperature of a portion of the second arm.

In one embodiment, the first semiconductor optical amplifier includes awaveguide in a first semiconductor chip; and the wavelength sensorincludes a photodiode, the photodiode being in the first semiconductorchip.

In one embodiment, a multiplexed multi-channel laser source includes: afirst multi-channel laser source, a second first multi-channel lasersource, and a multiplexer, the multiplexer including: a first input, asecond input, and an output, the multiplexer being configured: totransmit light from first input to the output, and to transmit lightfrom second input to the output.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated and understood with reference to the specification, claims,and appended drawings wherein:

FIG. 1A is a schematic illustration of a multichannel laser, accordingto an embodiment of the present invention;

FIG. 1B is a schematic illustration of an optical spectrum, according toan embodiment of the present invention;

FIG. 2A is a schematic illustration of a vernier ring resonator filter,according to an embodiment of the present invention;

FIG. 2B is a schematic illustration of a vernier ring resonator filter,according to an embodiment of the present invention;

FIG. 2C is a schematic illustration of a vernier ring resonator filter,according to an embodiment of the present invention;

FIG. 3A is a schematic illustration of a multichannel laser source,according to an embodiment of the present invention;

FIG. 3B is a schematic illustration of an optical spectrum, according toan embodiment of the present invention;

FIG. 4 is a schematic illustration of a multichannel laser source,according to an embodiment of the present invention;

FIG. 5 is a schematic illustration of a system for wavelength sensing,according to an embodiment of the present invention;

FIG. 6 is a schematic illustration of a multichannel laser source,according to an embodiment of the present invention;

FIG. 7 is a schematic illustration of a multichannel laser source,according to an embodiment of the present invention;

FIG. 8 is a schematic illustration of a multichannel laser source,according to an embodiment of the present invention;

FIG. 9A is a schematic illustration of a tunable grating-assistedco-directional coupler of a first type, according to an embodiment ofthe present invention;

FIG. 9B is a schematic illustration of a tunable grating-assistedco-directional coupler of a second type, according to an embodiment ofthe present invention; and

FIG. 9C is a schematic illustration of a tunable grating-assistedco-directional coupler of a third type, according to an embodiment ofthe present invention.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of amultichannel laser source provided in accordance with the presentinvention and is not intended to represent the only forms in which thepresent invention may be constructed or utilized. The description setsforth the features of the present invention in connection with theillustrated embodiments. It is to be understood, however, that the sameor equivalent functions and structures may be accomplished by differentembodiments that are also intended to be encompassed within the spiritand scope of the invention. As denoted elsewhere herein, like elementnumbers are intended to indicate like elements or features.

Referring to FIG. 1A, in a first embodiment, multiple RSOAs 100 ofdifferent band-gaps and material compositions are integrated toconstruct a multichannel tunable laser. Each RSOA provides optical gainto one or more of a plurality of laser channels, each generating lightat a respective wavelength, and all channels are combined into a singleoutput waveguide, or “bus waveguide” 115. In this embodiment, a singlevernier ring resonator tunable filter set (or simply “vernier ringresonator filter”) is employed, for each channel, to perform gainselection inside the laser cavity to select the lasing wavelength, andto combine the lasing light with that of other channels into the singlebus waveguide 115, all inside a laser cavity having a shared outputcoupler (or “output mirror”) 120. Each vernier ring resonator filteroperates as a wavelength-dependent coupler, as discussed in furtherdetail below. Only one vernier ring resonator filter is needed for eachchannel, and as many channels can be added as desired, at the expense ofmaking the laser cavity longer. Each RSOA chip may be separatelyfabricated (e.g., out of a III-V semiconductor material) and alignedwith (and bonded to) a silicon photonics chip which includes the otheroptical components shown (e.g., the vernier ring resonator filters 110,the bus waveguide 115, and the output coupler 120). The reflectivesurface of the RSOA may operate as the back mirror of the laser cavity.In some embodiments, the back mirror is a separate element from thesemiconductor optical amplifier. FIG. 1B shows an example of an opticalspectrum that may be generated by the embodiment of FIG. 1A. A fourthport of each vernier ring resonator filter may be connected to anoptical absorber 145 (or used for wavelength sensing), as discussed infurther detail below.

Referring to FIG. 2A, in some embodiments, each vernier ring resonatorfilter is a four-port device including three multimode interferencecouplers (MMIs) 210A-C, connected together with waveguide paths referredto as ring-halves 220A-220D. Each MMI has four ports that may bereferred to as an input port 230A, an output port 230B, a coupled port230C, and an isolated port 230D. The ratio of (i) power transmitted tothe coupled port to (ii) power supplied to the input port may bereferred to as the coupling factor. If the output port 230B is definedto be the port opposite the input port 230A, as in FIG. 2A, then thecoupling factor may also be referred to as the “cross coupling” and theratio of (i) the power transmitted to the coupled port to (ii) powersupplied to the input port may be referred to as the “bar coupling” (theterminology employed in the '078 Patent). The input port and theisolated port may be referred to (e.g., in the '078 Patent) as “backend” ports and the output port and the coupled port may be referred to(e.g., in the '078 Patent) as “front end” ports. Other examples ofvernier ring resonator filters are disclosed in the '078 Patent.

Each pair of ring-halves forms, with two respective MMIs, a closedoptical path that may be referred to as a ring resonator (or as a“simple” or “single” resonator, to distinguish it from a vernier ringresonator filter, which may include two or more (coupled) simple ringresonators). Light may be coupled into or out of this closed opticalpath through the MMIs. For example, for a first resonator including thefirst MMI 210A, the first and second ring halves 220A, 220B, and thesecond MMI 210B, light propagating in the forward direction (from theRSOA to the output coupler) may enter the input port 230A of the firstMMI 210A, and propagate around a first ring, formed by the first andsecond ring halves 220A, 220B and the first and second MMIs 210A, 210B.A portion of the light propagating around this ring may be coupled outof the first ring and into a second ring formed by the third and fourthring halves 220C, 220D and the second and third MMIs 210B, 210C. At awavelength for which the round-trip phase around the closed path is amultiple of 2π, the ring resonator may be said to be resonant, and lightcoupled into the ring resonator interferes constructively with lightcirculating in the ring resonator, resulting in greater circulatingpower than is the case for wavelengths at which the ring resonator isnot resonant. The wavelength (or frequency) separation betweenconsecutive resonant wavelengths of the ring resonator is referred to asa the free spectral range (FSR) of the ring resonator. The second ringresonator, formed by the third and fourth ring halves 220C, 220D and thesecond and third MMIs 210B, 210C, may operate in a similar manner.

The vernier ring resonator filter (which includes the first ringresonator and the second ring resonator) may, as mentioned above, act asa wavelength-dependent coupler, with (for example, for forwardtravelling light) the input port 230A of the first MMI 210A being theinput port of the vernier ring resonator filter, and the coupled port230C of the third MMI 210C being the coupled port of the vernier ringresonator filter. At a wavelength at which both the first resonator andthe second resonator are resonant, the vernier ring resonator filter isresonant, and the coupling ratio of the vernier ring resonator filter ishigh. The vernier ring resonator filter may be a reciprocal device, sothat when it is resonant, light returning, on the bus waveguide 115,from the output coupler 120, may be coupled, through the vernier ringresonator filter 110, back to the RSOA. When a resonant wavelength ofthe first vernier ring resonator filter is nearly equal to a resonantwavelength of the second vernier ring resonator filter, the vernier ringresonator filter may be resonant at a wavelength that is between the twowavelengths. When the vernier ring resonator filter is not resonant, thecoupling ratio of the vernier ring resonator filter is low. The firstring resonator and the second ring resonator may have slightly differentfree spectral ranges, so that the wavelengths at which the vernier ringresonator filter is resonant are relatively widely separated, and sothat only one wavelength at which the vernier ring resonator filter isresonant falls within the gain bandwidth of the RSOA. This (relativelywide) separation between consecutive resonant wavelengths of the vernierring resonator filter may be referred to as the free spectral range(FSR) of the vernier ring resonator filter. The width of a resonant peakof the vernier ring resonator filter (e.g., the wavelength range overwhich the coupling ratio is within 3 dB of the maximum coupling ratio inthe peak) may be referred to as the “bandwidth” of the vernier ringresonator filter, and it may be expressed in units of wavelength orfrequency. In some embodiments, a two-ring vernier ring resonator filtersuch as that of FIG. 2B, or a three-ring vernier ring resonator filtersuch as that of FIG. 2C, may be used instead of the two-ring vernierring resonator filter of FIG. 2A in the embodiment of FIG. 1A (or in theembodiments of FIGS. 3A, 4, and 5, discussed below). The resonantwavelength of a vernier ring resonator filter may be tuned using phaseshifters 135 (FIG. 1A) which may use heaters, as shown for example inFIGS. 2B and 2C, or which may use p-n or p-i-n junctions, as described,for example, in the '078 Patent. In some embodiments each of the MMIsshown in FIGS. 2A-2C has a cross coupling ratio equal to its barcoupling ratio.

Referring again to FIG. 1A, a plurality of vernier ring resonatorfilters 110 according to FIG. 2A (or alternate embodiments of thevernier resonator, such as those disclosed in the the '078 Patent) maybe used to couple a respective plurality of RSOAs to a bus waveguide, atthe output end of which a broadband partially reflective element acts asthe output coupler 120 for all of the channels. Each RSOA chip 100 mayprovide optical gain in a respective wavelength band. The RSOA chips 100may include different respective epitaxial (“epi”) designs or materialsystems, and have different respective gain spectrum center wavelengths,spanning, together, a large spectral range. The number of channels andthe bandwidth in each band may be limited by the gain bandwidth of theRSOA chip 100 for that band. Each RSOA chip may include an array ofRSOAs 105, each RSOA 105 being formed as a separate waveguide in theRSOA chip 100, and each providing optical gain for a respective one ofthe channels using the RSOA chip 100. As used herein, a “band” or“wavelength band” refers to a range of wavelengths over which an RSOAchip has appreciable gain.

In some embodiments, each channel includes a phase shifter 130 (Δφ) andan amplitude modulator 140 (ΔT). Phase shifters 130 may be included toenable accurate control of lasing wavelengths, and amplitude modulators140 may be included to enable modulation of the laser power. Thebandwidth over which amplitude modulation inside laser cavity may beperformed is inversely proportional to the cavity length. In someembodiments, amplitude modulation at rates of a few kHz, or a few MHz,may be used for channel identification or for homodyne/heterodynedetection at a receiver; modulation at GHz frequencies may beimpractical, in some embodiments, because of the length of the cavity.

In some embodiments, as an alternative to the use of amplitudemodulators, the RSOA bias is modulated with the desired amplitudemodulation pattern. This eliminates the need for separate amplitudemodulators inside the laser cavity which add loss, but increases thecomplexity of the RSOA drive circuitry. The length of the laser cavitymay be roughly the same for all channels, and increases for all channelsas more channels are added. The cavity length may be selected so thatthe wavelength separation between cavity modes (the free spectral rangeof the cavity) is greater than the bandwidth of any of the vernier ringresonator filters, so that only one mode at a time will lase in anychannel. For example, for a cavity free spectral range of 10 picometers(pm), a cavity length of about 5 cm may be used; this cavity length mayaccommodate 100 channels or more.

The output mirror may, for example, be implemented with a 1×2 powersplitter with a broadband high reflector on one output arm, where thesplit ratio of the 1×2 power splitter determines the reflectance of theoutput mirror, and where the splitter is implemented with a broadbandMMI or a directional coupler, and the broadband high reflector isimplemented with a metal coating, or a Sagnac loop. The total spectralspan of the multichannel laser may ultimately be limited by thecharacteristics of the broadband MMI or coupler used in the outputmirror. In some embodiments, the broadband output mirror is implementedwith an advanced thin film coating integrated in the output waveguide,or with a broadband (e.g., chirped) DBR grating included in the outputwaveguide.

Referring to FIG. 3A, in a second embodiment a multichannel laser sourceincludes a plurality of multichannel lasers each having a respective buswaveguide. A respective broadband partially reflective element acts, ineach multichannel laser, as a respective output coupler 120 for all ofthe channels of the multichannel laser. The outputs of all of themultichannel lasers are combined in a band multiplexer (MUX), to formthe output of the multichannel laser source. The embodiment of FIG. 3Amay be used if the bandwidth of a readily available output coupler 120(as used, for example, in the embodiment of FIG. 1A) is not sufficientto cover a desired spectral span, or if the cavity length would be toolong if the multichannel laser source were constructed according to FIG.1A. The embodiment of FIG. 3A may be more compact than the embodiment ofFIG. 1A, if the band MUX is compact. FIG. 3B shows an example of anoptical spectrum that may be generated by the embodiment of FIG. 3A.

As used herein, a multichannel laser refers to a laser having aplurality of channels, such as the laser of FIG. 1A, and being capableof producing light at more than one wavelength simultaneously, thechannels sharing at least one element (e.g., the output coupler 120 ofFIG. 1A). As used herein, a multichannel laser source refers to a lightsource that is capable of producing light at more than one wavelengthsimultaneously, and that includes one or more lasers (such as theembodiment illustrated in FIG. 3A, which includes three multichannellasers).

FIG. 3A shows one bus waveguide for each band, i.e., for each of theRSOA chips 100, but this correspondence is not required. Each buswaveguide may collect light from a plurality of channels using a singleRSOA chip 100 as illustrated, or it may collect light from a pluralityof channels using more than one different RSOA chip 100 (e.g., it maycollect light from one or more channels using a first RSOA chip 100 andfrom one or more channels using a second RSOA chip 100), or each of aplurality of bus waveguides may collect light from a respective subsetof a plurality of channels using a single RSOA chip 100.

Referring to FIG. 4, a third embodiment may be more compact than theembodiments of FIGS. 1A and 3A, if an inhomogeneously broadened RSOAmaterial is used (such as quantum dot (QD) or quantum dash (QDASH)material) in one or more of the RSOA chips 100 instead of a quantum well(QW) heterostructure. Inhomogeneously broadened gain materials cansupport lasing of multiple modes in a single waveguide (i.e., in asingle RSOA) in the RSOA chip 100. The wavelength spacing betweenchannels sharing an RSOA in the QD or QDASH RSOA chip may be larger thanthe homogeneous broadening width of the RSOA material, so that thechannels do not compete for gain to a significant extent. QD or QDASHmaterials may not be readily available for all bands of the multichannellaser source; accordingly some of the RSOA chips 100 may use suchmaterials, and some others may use other materials. The RSOA chips thatare inhomogeneously broadened may then have more than one channel perRSOA (i.e., per waveguide in the RSOA chip). In the embodiment of FIG.4, the top-most RSOA chip (used for the “p” wavelength band) isinhomogeneously broadened and includes only one RSOA 105, which supportsmultiple channels (of which three are shown) simultaneously; the othertwo RSOA chips 100 are configured to have only one channel per RSOA.FIG. 3B shows an example of an optical spectrum that may be generated bythe embodiment of FIG. 4.

It will be understood that in some embodiments, single ring resonators,or composite ring resonators including more than two ring resonators,may be used in place of one or more of the vernier ring resonatorfilters 110 of the embodiments of FIGS. 1A, 3A and 4. It will beunderstood that although only three RSOA chips and three bands areillustrated in FIGS. 1A, 3A, and 4, a multichannel laser or multichannellaser source may include more or fewer RSOA chips and bands.

Referring to FIG. 5, in some embodiments wavelength sensing may beperformed using an unequal-arm Mach-Zehnder interferometer 510 and twophotodetectors 520. Each of the photodetectors 520 may include areverse-biased junction on or coupled to a waveguide, and may befabricated on an RSOA chip 100 operating in the same wavelength band.The Mach-Zehnder interferometer 510 includes a first MMI 530 that actsas a splitter, and a second MMI 530 that acts as a combiner. Thewaveguide of a first arm 540 of the Mach-Zehnder interferometer 510 maylie alongside the waveguide of the second arm 550 of the Mach-Zehnderinterferometer 510, except for a portion 560 that may be longer than acorresponding portion of the second arm 550 and may result in the lengthdifference between the two arms 540, 550. The temperature of theMach-Zehnder interferometer 510 may be sensed and controlled, to reducethe differential phase change in the Mach-Zehnder interferometer 510that otherwise may occur if the temperature (and, as a result, thedifferential optical path delay) of the Mach-Zehnder interferometer 510were to change. FIG. 5 shows two channels of a multichannel laser sourcewhich has more than two channels (the remainder of which are not shownin FIG. 5).

The lengths of the two arms 540, 550 may be selected so that when thewavelength of light received by the Mach-Zehnder interferometer 510 isthe desired wavelength, the respective photocurrents generated by thetwo photodetectors 520 are equal. Accordingly, a feedback circuit mayform an error signal by calculating (e.g., using a differentialamplifier) the difference between two photocurrents, and the errorsignal may be amplified and filtered and fed back to one or moreelements (or “wavelength actuators”) for adjusting the wavelength. Sucha wavelength actuator may be part of a wavelength-dependent coupler (andmay be referred to as a “coupler wavelength actuator”) and may be, forexample, a phase shifter (e.g., a heater, or a p-i-n junction) on one ormore of (e.g., on all of) the half-rings, on a tunable grating-assistedco-directional coupler (discussed in further detail below) and/or on adistributed Bragg reflector (discussed in further detail below). In someembodiments, if the free spectral range of the laser cavity of a channelis greater than the resonant bandwidth of the wavelength-dependentcoupler, the phase shifter 130 may be controlled so as to keep aresonant frequency of the laser cavity within the resonant bandwidth ofthe wavelength-dependent coupler. In such an embodiment, the phaseshifter 130 acts as an additional wavelength actuator that may simplyfollow the center wavelength of the wavelength-dependent coupler, orthat may provide finer (or faster) wavelength control than the phaseshifter of the wavelength-dependent coupler. In this manner each outputwavelength may be controlled. Each of the output wavelengths may also betunable, for example by adding an offset signal to the error signalbefore it is amplified and filtered by the feedback circuit. TheMach-Zehnder interferometer 510 may be fed a portion of the lightemitted by the RSOA of the channel for which the wavelength is to bemeasured, e.g., it may be fed by light from the output port 230B of thefirst MMI 210A of a respective vernier ring resonator filter of thechannel for which the wavelength is to be measured (and controlled), asshown in FIG. 5.

FIG. 6 shows an embodiment of a multichannel laser that is analogous tothe embodiment of FIG. 1A, in which the vernier ring resonator filters110 of FIG. 1A have been replaced with grating-assisted co-directionalcouplers (e.g., tunable grating-assisted co-directional couplers(TGACDCs)) of a first type. These grating-assisted co-directionalcouplers operate as wavelength-dependent couplers, as discussed infurther detail below. In a manner analogous to that of the embodiment ofFIG. 1A, in the embodiment of FIG. 6 a TGACDC is employed, for eachchannel, to perform gain selection inside the laser cavity to select thelasing wavelength, and to combine the lasing light with that of otherchannels into the single bus waveguide 115, all inside a laser cavityhaving a shared output coupler 120. FIG. 1B shows an example of anoptical spectrum that may be generated by the embodiment of FIG. 6.

FIGS. 7 and 8 show embodiments of multichannel laser sources eachincluding a plurality of channels, using TGACDCs as bothwavelength-selective output couplers and as couplers for combining thelight generated by the plurality of channels. FIGS. 7 and 8 use TGACDCsof a second and third type, respectively, as discussed in further detailbelow.

Three different types of TGACDCs may be used: i) a first type (asdescribed in Z.-M. Chuang and L. A. Coldren, IEEE JQE 29 (4) 1993 p.1071) designed to 100% transmit distributed Bragg reflector (DBR)resonant wavelengths to the drop T port (FIG. 9A, “TGACDC1”), ii) asecond type designed to partially reflect DBR resonant wavelengths backto the RSOA (FIG. 9B, “TGACDC2”), and partially transmit resonantwavelengths to the drop T port, and iii) a third, composite type,consisting of a regular DBR (as described in A. J. Zilkie et al.,“Power-efficient III-V/Silicon external cavity DBR lasers,” Opt. Express20, 23456-23462 (2012)) combined with a TGACDC of the first type, toform a composite device (FIG. 9C, “TGACDC3”), also designed to partiallyreflect DBR resonant wavelengths (wavelengths that are resonant both inthe DBR and in the included TGACDC of the first type) back to the RSOA,partially transmit resonant wavelengths to the following TGACDC of thefirst type, and further 100% transmit (by the TGACDC of the first type)the resonant wavelengths to the drop T port. A composite TGACDC of thethird type may behave qualitatively like a TGACDC of the second type,and may be used instead of a TGACDC of the second type to avoiddifficult design constraints that may be present in a TGACDC of thesecond type. Off resonance (i.e., for wavelengths for which the TGACDCis not resonant) each of the three types TGACDC may behave as twosubstantially independent, parallel waveguides, with little or nocoupling between them (i.e., light passing straight through the topwaveguide with little or no coupling to the drop waveguide), and littleor no reflection from the TGACDC.

In some embodiments all grating DBR wavelengths are made tunable byadding a waveguide integrated heater to the grating (e.g., using a metalon waveguide heater or a Si-doped integrated heater, possibly with anundercut to make it more efficient).

As mentioned above, if a TGACDC of the first type (the type of FIG. 9A)is used (as illustrated in FIG. 6, for example), each TGACDC performsthe same function as one of the vernier ring resonator filters 110 ofFIG. 1A, and all of the lasers (at different respective wavelengths)share an output mirror, the output mirror for all of the lasers beingthe common output mirror 120 shown.

Referring to FIG. 7, if a TGACDC of the second type 710 (the type ofFIG. 9B) is used, the TGACDC 710 acts like a regular DBR mirror buttransmits laser output light out to the bus waveguide. In this case nocommon output mirror is needed as each wavelength's output mirror is therespective TGACDC 710. This also provides the advantage that the cavitylength for the respective laser at each wavelength is much shorter,close to a traditional single channel DBR, meaning that the cavity FSRis much larger, and each laser may therefore have a much larger tuningrange between mode hops, and the number of lasers that can be added isnot limited. A suitable TGACDC 710 of the type of FIG. 9B may however bemore difficult to design and may have more design constraints and/or bemore difficult to fabricate. FIG. 1B shows an example of an opticalspectrum that may be generated by the embodiment of FIG. 7.

A TGACDC of the third type (the type of FIG. 9C) is an alternativeimplementation to the TGACDC of the type of FIG. 9B, but would havedesign constraints and/or fabrication difficulties relaxed because theDBR that acts as the laser mirror is separated from the GACDCfunctionality. FIG. 8 shows a multichannel laser source using TGACDCs ofthe third type 810. In FIG. 8 each composite TGACDC of the third type810 is illustrated for simplicity as a simple TGACDC. FIG. 1B shows anexample of an optical spectrum that may be generated by the embodimentof FIG. 8. Although the grating-assisted co-directional couplers are, inthe descriptions of some embodiments, referred to as TGACDCs (“tunablegrating-assisted co-directional couplers”), in some embodiments thegrating-assisted co-directional couplers are not tunable.

In each of FIGS. 6, 7, and 8, wavelength sending and control may beperformed by feeding a portion of the light emitted by the RSOA of thechannel for which the wavelength is to be measured to a suitablewavelength sensor (as described, for example, with reference to FIG. 5),e.g., the wavelength sensor may be fed a portion of the power out of the“out 1 through” port of the TGACDC. In other embodiments this port ofthe TGACDC is coupled to an optical absorber to prevent parasiticcavities or light from this port being back-reflected into the lasercavity or the bus waveguide.

As used herein, a “wavelength-dependent coupler” is an optical devicewith at least three ports, including a channel port, a bus input, and abus output, and in which the coupling between ports, or the reflectanceof one or more ports, depends on the wavelength of light fed to thewavelength-dependent coupler. The ports of the wavelength-dependentcoupler may also be referred to by other names, as, for example, in thedescriptions above of vernier ring resonator filters and ofgrating-assisted co-directional couplers. In some embodiments (e.g.,those of FIG. 1A and FIG. 6), light fed to the channel port at aresonant wavelength of the wavelength-dependent coupler may be largelytransmitted to the bus output of the wavelength-dependent coupler (e.g.,with loss of between 0.1 dB and 1.0 dB, or of less than 0.1 dB), andlight fed to the bus input at a wavelength different from the resonantwavelength of the wavelength-dependent coupler (or different from everyresonant wavelength of the wavelength-dependent coupler, if it has morethan one resonant wavelength) may be largely transmitted (e.g., withloss of between 0.1 dB and 1.0 dB, or of less than 0.1 dB) to the busoutput of the wavelength-dependent coupler. The reflectance back to thechannel port and the reflectance back to the bus input may be between10% and 1%, e.g., less than 5% or even less than 1%. In otherembodiments (e.g., those of FIGS. 7 and 8), light fed to the channelport at a resonant wavelength of the wavelength-dependent coupler may bepartially reflected (e.g., with a reflectance of between 10% and 50%),and partially transmitted (e.g., with a transmittance of between 40% and90%) to the bus output of the wavelength-dependent coupler, and lightfed to the bus input at a wavelength different from the resonantwavelength of the wavelength-dependent coupler (or different from everyresonant wavelength of the wavelength-dependent coupler, if it has morethan one resonant wavelength) may be largely transmitted (e.g., withloss of between 0.1 dB and 1.0 dB, or of less than 0.1 dB) to the busoutput of the wavelength-dependent coupler. The reflectance (i) back tothe channel port, at a wavelength different from the resonant wavelengthof the wavelength-dependent coupler (or different from every resonantwavelength of the wavelength-dependent coupler, if it has more than oneresonant wavelength) and (ii) back to the bus input, may be between 10%and 1%, e.g., less than 5% or even less than 1%. A wavelength-dependentcoupler may have a fourth port that may (as mentioned above) beconnected to an optical absorber or that may be used as a source of aportion of the light generated in one of the channels, for use in awavelength sensing and control system for the channel. Both possibleuses of the fourth port are illustrated in FIGS. 6, 7, and 8, witharrows at the fourth ports of two of the wavelength-dependent couplersdenoting light sent to a wavelength sensor. In some embodiments, the 3dB bandwidth of the resonant characteristic of a wavelength-dependentcoupler is between 0.01 nm and 1.00 nm.

As mentioned above, the vernier ring resonator filters (e.g., of FIG.1A) and the grating-assisted co-directional couplers (e.g., TGACDCs)described herein are examples of wavelength-dependent couplers. Forexample, in FIG. 1A the channel port of the each of the vernier ringresonator filters is the upper left port, the bus input is the lowerleft port, the bus output is the lower right port, and the fourth portis the upper right port.

Any numerical range recited herein is intended to include all sub-rangesof the same numerical precision subsumed within the recited range. Forexample, a range of “1.0 to 10.0” is intended to include all subrangesbetween (and including) the recited minimum value of 1.0 and the recitedmaximum value of 10.0, that is, having a minimum value equal to orgreater than 1.0 and a maximum value equal to or less than 10.0, suchas, for example, 2.4 to 7.6. Any maximum numerical limitation recitedherein is intended to include all lower numerical limitations subsumedtherein and any minimum numerical limitation recited in thisspecification is intended to include all higher numerical limitationssubsumed therein.

Although exemplary embodiments of a multichannel laser source have beenspecifically described and illustrated herein, many modifications andvariations will be apparent to those skilled in the art. Accordingly, itis to be understood that a multichannel laser source constructedaccording to principles of this invention may be embodied other than asspecifically described herein. The invention is also defined in thefollowing claims, and equivalents thereof.

What is claimed is:
 1. A multi-channel laser source, comprising: a buswaveguide coupled, at an output end of the bus waveguide, to an outputof the multi-channel laser source; a first semiconductor opticalamplifier; a first back mirror; a first wavelength-dependent couplerhaving a first resonant wavelength; a second semiconductor opticalamplifier; a second back mirror; and a second wavelength-dependentcoupler having a second resonant wavelength, different from the firstresonant wavelength; the first semiconductor optical amplifiercomprising: a first end coupled to the first back mirror, and a secondend, the first wavelength-dependent coupler comprising: a channel portconnected to the second end of the first semiconductor opticalamplifier; a bus output connected to a first portion of the buswaveguide; and a bus input, connected to a second portion of the buswaveguide more distant from the output end of the bus waveguide than thefirst portion of the bus waveguide; the second semiconductor opticalamplifier being coupled to the bus waveguide through the secondwavelength-dependent coupler, the first wavelength-dependent couplerbeing nearer to the output end of the bus waveguide than the secondwavelength-dependent coupler, the first wavelength-dependent couplerbeing configured to transmit light, at the second resonant wavelength,from the bus input of the first wavelength-dependent coupler to the busoutput of the first wavelength-dependent coupler.
 2. The multi-channellaser source of claim 1, further comprising an output coupler at theoutput end of the bus waveguide, wherein the first wavelength-dependentcoupler is configured to transmit light at the first resonant wavelengthfrom the channel port of the first wavelength-dependent coupler to thebus output of the first wavelength-dependent coupler.
 3. Themulti-channel laser source of claim 1, wherein the firstwavelength-dependent coupler is configured to reflect a first portion oflight received at the first resonant wavelength at the channel port ofthe first wavelength-dependent coupler, and to transmit, to the busoutput of the first wavelength-dependent coupler, a second portion oflight received at the first resonant wavelength at the channel port ofthe first wavelength-dependent coupler.
 4. The multi-channel lasersource of claim 3, wherein the first portion is at least 10% of thelight received, and the second portion is at least 40% of the lightreceived.
 5. The multi-channel laser source of claim 1, wherein thefirst wavelength-dependent coupler is configured to transmit, to afourth port of the first wavelength-dependent coupler, light received atthe channel port at the second resonant wavelength.
 6. The multi-channellaser source of claim 5, wherein the fourth port of the firstwavelength-dependent coupler is connected to an optical absorber.
 7. Themulti-channel laser source of claim 1, wherein the first back mirror andthe first semiconductor optical amplifier are configured as a reflectivesemiconductor optical amplifier.
 8. The multi-channel laser source ofclaim 1, wherein the first wavelength-dependent coupler comprises afirst ring resonator.
 9. The multi-channel laser source of claim 8,wherein the first wavelength-dependent coupler further comprises asecond ring resonator, the first ring resonator and the second ringresonator being configured to operate as a vernier ring resonatorfilter.
 10. The multi-channel laser source of claim 1, wherein the firstwavelength-dependent coupler comprises a grating assisted co-directionalcoupler.
 11. The multi-channel laser source of claim 10, wherein thefirst wavelength-dependent coupler further comprises a distributed Braggreflector connected in cascade with the grating assisted co-directionalcoupler.
 12. The multi-channel laser source of claim 1, wherein thefirst wavelength-dependent coupler comprises a wavelength actuator foradjusting the first resonant wavelength.
 13. The multi-channel lasersource of claim 1, further comprising a phase shifter between the firstback mirror and the first wavelength-dependent coupler.
 14. Themulti-channel laser source of claim 1, further comprising an amplitudemodulator between the first back mirror and the firstwavelength-dependent coupler.
 15. The multi-channel laser source ofclaim 1, wherein the first semiconductor optical amplifier is the samesemiconductor optical amplifier as the second semiconductor opticalamplifier.
 16. The multi-channel laser source of claim 1, wherein thefirst semiconductor optical amplifier comprises a first waveguide in afirst semiconductor chip and the second semiconductor optical amplifiercomprises a second waveguide in the first semiconductor chip.
 17. Themulti-channel laser source of claim 1, wherein the first semiconductoroptical amplifier comprises a waveguide in a first semiconductor chip,and the second semiconductor optical amplifier comprises a waveguide ina second semiconductor chip, different from the first semiconductorchip.
 18. The multi-channel laser source of claim 1, further comprising:a wavelength sensor configured to receive a portion of, and to sense awavelength of, light emitted by the first semiconductor opticalamplifier; and a control system configured: to receive a wavelengthsensing signal from the wavelength sensor, to calculate a differencebetween the wavelength sensing signal and a wavelength setpoint, and toapply a wavelength correction signal to a wavelength actuator, to reducethe difference between the wavelength sensing signal and the wavelengthsetpoint.
 19. The multi-channel laser source of claim 18, furthercomprising a phase shifter between the first back mirror and the firstwavelength-dependent coupler, wherein the wavelength actuator comprisesthe phase shifter.
 20. The multi-channel laser source of claim 18,wherein the first wavelength-dependent coupler comprises a couplerwavelength actuator for adjusting the first resonant wavelength, whereinthe wavelength actuator comprises the coupler wavelength actuator. 21.The multi-channel laser source of claim 18, wherein the wavelengthsensor is configured to receive light from a fourth port of the firstwavelength-dependent coupler.
 22. The multi-channel laser source ofclaim 18, wherein the wavelength sensor comprises a Mach-Zehnderinterferometer having a first arm and a second arm, longer than thefirst arm, and a temperature control system configured to control thetemperature of a portion of the second arm.
 23. The multi-channel lasersource of claim 18, wherein: the first semiconductor optical amplifiercomprises a waveguide in a first semiconductor chip; and the wavelengthsensor comprises a photodiode, the photodiode being in the firstsemiconductor chip.
 24. A multiplexed multi-channel laser sourcecomprising: a first multi-channel laser source according to claim 1, asecond first multi-channel laser source according to claims 1, and amultiplexer, the multiplexer comprising: a first input, a second input,and an output, the multiplexer being configured: to transmit light fromfirst input to the output, and to transmit light from second input tothe output.